Newly disclosed details of the millions of dollars flowing into political groups are highlighting not just the scale of donations from corporations and unions but also the secrecy surrounding super PACs seeking to influence the presidential race.

Some of the money came from well-established concerns, such as Alpha Natural Resources, one of the country’s largest coal companies, or from the Service Employees International Union, a powerful union allied with Democrats, according to filings with the Federal Election Commission.

Some came from companies closely identified with prominent industrialists or financiers, like Contran, a holding company controlled by Texas billionaire Harold Simmons, a patron of a number of conservative groups and candidates, and Blue Ridge Capital, a New York hedge fund founded by wealthy investor John Griffin, a supporter of Mitt Romney.

But some checks came from sources obscured from public view, like a $250,000 contribution to a super PAC backing Romney from a company with a post office box for a headquarters and no known employees.

President Barack Obama continues to outraise all of the candidates seeking the Republican nomination by large margins when it comes to money that goes directly into campaign coffers.

But the money race is increasingly focused on outside groups that are legally not allowed to coordinate directly with campaigns but pay for advertising and other activities that support particular candidates.

Much of the money disclosed this week went to independent groups supporting Republicans, giving them an enormous money advantage over similar Democratic groups in the first phase of the 2012 election cycle.

Such donations were made possible by the Supreme Court’s Citizens United decision in 2010 and subsequent court rulings, which opened the door to unlimited corporate and union contributions to political committees and made it possible to pool that money with unlimited contributions from wealthy individuals.

But the full scope of such giving is impossible to ascertain from federal campaign filings.

Much of the money raised by the leading Republican and Democratic independent groups went into affiliated nonprofit organizations that are more restricted in how they can spend the money but do not have to disclose their donors.

The contributions already have helped the Republican Party’s elite donor class, who increasingly favors Romney, regain some control over the party’s nominating process.

Many of the party’s top givers sent checks to Restore Our Future, the pro-Romney group, underwriting an advertising campaign that battered Romney’s Republican rivals even as Romney himself struggled to win the trust of the party’s restive conservative base.

Romney, who assailed Newt Gingrich in Florida last week for “working as a lobbyist and selling influence around Washington,” also got a major boost from some of the Republican Party’s top corporate lobbyists, who raised more than $1 million in checks for Romney’s campaign during the last three months of 2011.

Restore Our Future raised at least $5.8 million from corporations during the last six months of last year, along with $12.2 million from individuals.

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